1. Field of the Invention
The present invention relates to a silicide semiconductor device having a polycrystalline silicon layer sandwiched between a semiconductor substrate and a metal silicide layer.
2. Description of the Related Art
Generally, in a semiconductor device, a metal layer made of aluminum, for example, is deposited as an electrode on a semiconductor substrate made of silicon. In this case, when a heat process is carried out, the metal layer reacts on the semiconductor substrate to form a metal silicide layer. If a shallow PN junction is formed in the semiconductor substrate in advance, metal is immersed through the PN junction, so that the PN junction is broken down which invites electrical leakage.
In a prior art semiconductor device, in order to avoid the above-described electrical leakage caused by the breakdown of the PN junction, a polycrystalline silicon layer serving as an impurity diffusion source is interposed between the metal layer and the semiconductor substrate. That is, an insulating layer is formed on the semiconductor substrate, and an opening is perforated in the insulating layer. Then, a polycrystalline silicon layer is deposited on the semiconductor substrate through the opening of the insulating layer. In this case, impurity ions are doped into the polycrystalline silicon layer, and thereafter, a heat operation is performed upon the polycrystalline silicon layer, so that an inpurity doped region is formed in the semiconductor substrate. Thus, a shallow PN junction is formed within the semiconductor substrate.
Even in the above-described prior art semiconductor device. however, if the thickness of the insulating layer is too large, it is impossible to avoid breakdown of PN junctions due to the immersion of metal into the semiconductor substrate. This will be explained later in detail.